The present invention relates to luminaires generally, and more particularly relates to indirect luminaires which illuminate a space by reflecting light off interior ceiling and wall surfaces. Still more particularly the invention relates to an indirect luminaire employing a lens to spread the light directed from the luminaire.
The concept of indirect lighting has been known for many years. Conventionally a light source is hidden totally from view within an opaque luminaire housing. Light is directed generally upwardly and laterally out of a top opening in the housing toward an overhead ceiling or adjacent side wall surface, or both. The illuminated ceiling and/or wall surfaces thusly become the effective light source. In recent years attention has been given to controlling the distribution of light emitted by an indirect fixture by the addition of a lens or lenses to the fixture's top opening. The principal objective of such light control is to evenly spread the light over the reflecting wall/ceiling surfaces; another objective has to be to provide the psychological perception of seeing the light source in the visible part of the lens. For example, one form of indirect fixture lens is disclosed in U.S. Pat. No. 4,390,930 wherein a small controlled amount of light is directed below the horizontal plane of the fixture to give a hint of brightness to the lens for visually locating the light source while preventing excess brightness.
Recent use of lenses on indirect luminaires have been on what are known as linear lighting fixtures, which are fixtures having a lens and housing of a uniform cross-section throughout its length, and which are typically straight and elongated. The lenses, which are fabricated of an extruded plastic and have elongated linear prisms which run longitudinally of the lens, are removably secured to the top edge of the fixture housings. One of the pervasive problems in such linear fixtures has been the problem of excessively bright strips of light appearing along certain of the elongated prisms of the fixture's lens. Such excessive brightness is generally difficult to eliminate in all of the prisms, and their presence can produce enough discomforture to detract from the fixture's overall pleasing appearance. One recent solution to this excess brightness problem has been to mask certain normally occurring bright strips as disclosed in co-pending application Ser. No. 596,811 now U.S. Pat. No. 4,573,111.
Using conventional lens designs, the problem of excessive lens brightness due to overly bright prisms increases as the fixture's lamps are raised within the fixture's housing. Excessive brightness can generally be avoided by positioning the fixture's lamps well within the housing itself. If the lamps are raised to approximately the level of the top of the housing side walls, it has been found that excessive lens brightness will often occur at certain horizontal angles as the viewer walks around the fixture, normally at side angles as opposed to a 90.degree. angle where the leans of the fixture is being looked at directly from the side. These high brightness area are sometimes referred to as "hot spots" or "flashes" on the lens. Elevating the lamp position still further generally exacerbates the problem of hot spots, and can create a condition where excessive brightness appears in the prismatic lens for all horizontal angles.
In a lensed indirect fixture it would be advantageous to elevate the light source to a position above the top of the housing so that more light from the lamp passes directly through the lens. This, in many cases, will achieve increased fixture efficiency and will also permit another discovered advantage in a two way lighting design mentioned below.
In the present invention a prismatic lens of an indirect luminaire has been devised which will take the light from an elevated light source, that is, a light source positioned substantially above the top of the fixture housing, and redirect the light near or above the luminaire's horizontal plane so as to eliminate excess brightness in the lens for all horizontal angles of viewing. It has specifically been discovered that excessive brightness which occurred in previous lens designs was caused in large part by light transmitted through the lens from a horizontal side angle relative to the lens surface, as opposed to light having a 90.degree. or perpendicular angle of incidence to the lens. Thus, the present invention specifically provides an indirect luminaire having a lens which solves the problem of excessive brightness caused by side angle light.
As above mentioned, the elevated light source of the invention provides another advantageous luminaire design. It is noted that heretofore fluorescent indirect luminaires have typically been provided with one, two or three lamp configurations. The multi-lamp configurations have been achieved by placing the fluorescent lamps in a side-by-side relation. Such a side-by-side lamp configurations have in part been dictated by space considerations where the objective has been to position the lamps down within the fixture housing.
In normal operation all lamps of a multi-lamp fixture would be illuminated at the same time, however, in recent years it has become desirable to design lighting fixtures with two level lighting wherein the fixture can be switched to turn on all lamps or less than all lamps. Such two-level lighting permits the control of light level at different times of the day for energy conservation purposes. The difficulty with such two-level lighting in a symmetrical indirect luminaire having a light spreading lens is that the low level of the two-level lighting, where a single off center lamp would typically be illuminated, will normally cause the source of light to be asymmetric relative to the lens thereby disturbing the overall light distribution pattern of the luminaire. Therefore, as the luminaire is switched from one level to the next the performance of the fixture in terms of light distribution visibly changes. It will be seen that a further aspect of the present invention is to provide a linear indirect luminaire with at least two switchable lamps wherein the light source is symmetric to the luminaire lens regardless of whether one lamp or both lamps are illuminated.